1. Field of the Invention
The present invention relates to a surface acoustic wave device in which a SiO2 film is laminated so as to cover a piezoelectric substrate and an IDT electrode. Particularly, the present invention relates to a surface acoustic wave device in which an IDT electrode is composed of a metal filled in grooves located in an upper surface of a piezoelectric substrate.
2. Description of the Related Art
A duplexer (DPX) or an RF filter used for a mobile communication system, such as a mobile phone, is required to have both wide band characteristics and superior temperature characteristics. Previously, a surface acoustic wave device has been widely used as a DPX or an RF filter.
In particular, for the application described above, a surface acoustic wave device in which an IDT electrode is formed on a piezoelectric substrate of LiTaO3 or LiNbO3 has been widely used. LiTaO3 and LiNbO3 each have a negative temperature coefficient of frequency TCF. Hence, a method for improving temperature characteristics has been known in which a SiO2 film having a positive temperature coefficient of frequency is formed on a piezoelectric substrate so as to cover an IDT electrode.
However, when a SiO2 film is formed, at boundaries each between a portion at which an electrode finger of the IDT electrode is present and a portion at which no electrode finger thereof is present, steps are inevitably formed on the surface of the SiO2 film. Accordingly, because of irregularities formed by the steps described above, an insertion loss is disadvantageously degraded.
In the following Japanese Unexamined Patent Application Publication No. 2004-112748, as a method for solving the problem described above, a method has been disclosed in which, after a first insulating layer having a thickness equal to that of an IDT electrode is formed between electrode fingers thereof, a SiO2 film is formed so as to cover the IDT electrode and the first insulating layer. Since the first insulating layer is formed, an underlayer for the SiO2 film becomes flat, so that the surface of the SiO2 film can be planarized.
According to the surface acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 2004-112748, the IDT electrode is formed of a single layer film composed of a metal having a density higher than that of Al or an alloy primarily including the above metal or is formed of a multilayer film composed of a film of a metal having a density higher than that of Al or an alloy primarily including the above metal and a film of another metal, and an electrode density of the IDT electrode is set to 1.5 times or more than that of the first insulating layer.
As described above, according to the surface acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 2004-112748, since the IDT electrode primarily composed of a metal heavier than Al is used as described above, variation in acoustic velocity and/or frequency with respect to variation in electrode thickness is liable to increase. On the other hand, when the IDT electrode is formed from Al, it has been known that a reflection coefficient of the IDT electrode considerably decreases, and hence sufficient characteristics as a surface acoustic wave resonator and/or a surface acoustic wave filter cannot be obtained.
As a device which solves the problem described above, in the following WO2006/011417A1, there has been disclosed a surface acoustic wave device which has an IDT electrode composed of Al filled in a plurality of grooves formed in an upper surface of a piezoelectric substrate of LiTaO3 or LiNbO3. In the surface acoustic wave device disclosed in WO2006/011417A1, Al is filled in the grooves, so that the IDT electrode is formed. Furthermore, a SiO2 film is laminated so as to cover the IDT electrode. Accordingly, since an underlayer for the SiO2 film is planarized, the surface of the SiO2 film can be planarized.
In addition, according to WO2006/011417A1, a LiTaO3 substrate having specific Euler angles or a LiNbO3 substrate having Euler angles (0°, 85° to 120°, 0°), (0°, 125° to 141°, 0°), (0°, 145° to 164°, 0°), or (0°, 160° to 180°, 0°) are preferably used, and a LiNbO3 substrate having Euler angles (0°, 90° to 110°, 0°), (0°, 125° to 136°, 0°), (0°, 149° to 159°, 0°), or (0°, 165° to 175°, 0°) is more preferably used.
In the structure in which a SiO2 film is laminated on an IDT electrode composed of Al in order to improve frequency temperature characteristics, the reflection coefficient described above decreases, and the characteristics are liable to be degraded. On the other hand, according to the structure disclosed in WO2006/011417A1, the IDT electrode composed of Al is embedded in the grooves provided in the piezoelectric substrate. Hence, the reflection coefficient of the electrode is made sufficiently high. Furthermore, since the SiO2 film is formed, the frequency temperature characteristics are improved. In addition, since the surface of the SiO2 film is planarized, the insertion loss is not likely to increase.